For problem 1:
When dividing with decimals, you move the decimal to the right until the number is whole.
EX:
20.8 ÷ 2.6 → 208 ÷ 26
Then you divide like normal.
(Answer is: 0.2)
Answer for 2: 1/4
Answer for 3: 45
Answers for 4: 1.8, 1.81, 1.810
Answers for 5: 642, 6420, 64200, 6.42, 0.642, 0.0642